From silicone resins and silicone rubbers, including polymers exhibiting physical properties between those of the silicone resin and rubber, there can be obtained products which are advantageous not only in that they have a heat resistance, a low-temperature resistance, electrical insulation properties and others, but also in that they are transparent. Therefore, the silicone resins and silicone rubbers are used in various optical applications. Particularly, in the applications of sealing or protection of LED and lens, a polyorganosiloxane composition which is cured to form a transparent cured product having high hardness is useful.
Patent document 1 discloses that a resin-form cured product, which is obtained by crosslinking branched polyorganosiloxane having an alkenyl group bonded to a silicon atom, particularly the polysiloxane having a phenyl group for achieving high refractive index, with polyorganohydrogensiloxane, is used in protection, bonding, or wavelength changing or controlling of LED, and lens. However, there is a problem in that, when the phenyl group content of the polyorganosiloxane is high, the resultant resin-form cured product has poor light transmission properties in a short wavelength region, i.e., in the blue to ultraviolet region. Further, the cured product has a drawback in that it suffers yellowing when exposed to heat. In recent years, in the field of LED, the luminance is improved, and the heat release increases as the luminance is increased. Hence, this problem must be solved as soon as possible.
On the other hand, as, a technique approaching the same task, which is different from introducing a phenyl group into polyorganosiloxane, patent document 2 discloses a resin-form cured product obtained by crosslinking polyorganosiloxane with polyorganohydrogensiloxane, wherein the polyorganosiloxane has a hydroxyl group or alkoxy group bonded to a silicon atom and has monovalent hydrocarbon groups bonded to a silicon atom wherein 80% or more of the monovalent hydrocarbon groups, excluding alkenyl groups, are methyl groups. However, there is a problem in that hydrolysis or polycondensation proceeds in the resultant resin-form cured product due to the remaining hydroxyl group or alkoxy group to cause a change with the lapse of time in hardness and others, so that the cured product becomes brittle.    Patent document 1: Japanese Unexamined Patent Publication No. 2004-186168    Patent document 2: Japanese Unexamined Patent Publication No. 2004-221308